grouped_parser_processor::grouped_parent_exists needs a cache, it can get 10,000,000 for 500 paths on restore

Restoring large numbers of questions in a restore results in the grouped_parser_processor::grouped_parent_exists getting called many millions of times. However due to the XML structure there are only a few parent paths to the top of the XML tree.

Removing the calls to progressive_parser::dirname($parentpath) and being able to complete simple cache lookups will reduce restore times significantly.